1. Field of the Invention
The present invention relates an electromagnetic clutch used, for example, for an air conditioning apparatus for an automobile, for selectively operating an compressor in the air conditioning apparatus.
2. Description of the Related Art
In an air conditioning apparatus for an automobile, an electromagnetic clutch is used for selectively transmitting the rotation of a crankshaft of the internal combustion engine to a compressor of the air conditioning apparatus. The electromagnetic clutch is provided with a rotor rotatable on a shaft of the compressor, a pulley fixed to the rotor, which pulley is kinematically connected to a crankshaft of the engine, and a armature connected to a hub of the shaft of the compressor. The armature has an engaging surface transverse to the axis of the shaft and facing, with a small gap therebetween, an end surface of the rotor acting as a frictional engaging surface. The pulley and the rotor have an overhanging relationship such that the pulley extends axially beyond the frictional engaging surface of the rotor.
The sliding surface of the rotor is roughened to thereby increase a frictional force obtained when the sliding surface is brought into the contact with the armature. To reduce production costs, this roughening process is carried out after the pulley and the rotor are welded. The overhanging of the pulley with respect to the sliding surface of the rotor makes it difficult to machine that surface with a grinding wheel, and accordingly, a bit is used, to make a tool mark on the sliding surface of the rotor, constructed by a succession of a plurality of sections having a substantially circular shape concentric to the axis of the rotation of the rotor. This concentric construction, however, does not produce a high friction factor, and thus the transmission torque is reduced.